I've lived in California all my life and was somehow ignorant of this fact. Apparently it does exist, although the bulk of the Urban Rail system is light rail, not grade separated. Daily ridership is somewhere around 360K (not bad honestly!) vs. SF Muni at 660K and NYC at 5.7MM.
The light rail isn't that bad as far as traffic goes: it isn't grade separated, but its not in the street either (e.g. to Santa Monica). The two subway lines I'm referring to (red and purple) are actually subway lines though (vs. the Expo line which is basically light rail). Living in LA, I actually never had a chance to ride any of these routes.